Concentric co-extrusion die and a method of extruding a multilayer thermoplastic film

ABSTRACT

A large concentric co-extrusion die ( 1 ) is described having a plurality of annular or conical die mandrel layers ( 201 - 205 ). Each layer is formed between a pair of adjacent annular or conical die mandrels ( 101 - 106 ) defining between them a flow path for molten thermoplastics material from an inlet to an annular extrusion outlet ( 110 ) through which a thermoplastics tubular extrusion is formed in use. Extrusion takes place through the multiple annular layer outlets ( 301 - 305 ) to form a multi-layered product. At least one layer ( 203 ) of the annular or conical die mandrels has a plurality of molten material inlets arranged around the external circumference of the co-extrusion die with each inlet being connected to a feed channel ( 403 ) which has plural bifurcations ( 403.1, 403.2, 403.3 ) providing 2 n  subsidiary outlet feed channels ( 503 ) where n is the number of bifurcations. Each subsidiary outlet feed channel being connected to a corresponding helical outlet channel ( 703 ).

The present invention relates to extrusion dies and, more particularly,to co-extrusion dies and especially those for extruding large widthblown films using thermoplastic materials.

BACKGROUND

The basic function of any blown film extrusion die is to take one ormore melt streams entering the die and distribute them to a singleconcentric annular melt stream at the die exit as uniformly as possible.

A number of different types of extruder die are known in the art.Concentric helical mandrel dies are cylindrical in shape and are mountedone above another secured to a common component to maintain theirrelative positions. Variations of this design may have, for example, acentral feed where all melt streams are fed to the centre and then splitto the helical outlet channels with radially arranged tubular ports. Anexample of this type is shown in U.S. Pat. No. 3,966,377. Anothervariation is an annular feed mandrel die in which the melt streams flowinto the die through centrally located concentric annuli and then,through outwardly radially extending tubular ports, to helical outletchannels. This variation can be implemented in two sub-variations; onewith central part of the feed block is open for IBC tube installation,the other having the inner layer stream as the central pipe of the feedblock. Examples are to be found in U.S. Pat. No. 4,182,603. A furthervariation is a side-feed mandrel die in which each layer has a singlemelt inlet on the outside of the die and the melt is then distributed ina pair of vertical paths to the entries of the helical outlet channels,each of the two distribution paths being on the same cylindrical surfaceas the corresponding helices. An example is to be found in U.S. Pat. No.7,811,073.

Conical stacked mandrels stack one over another, with a variation ofthis design consisting of conical shaped mandrels stacked over oneanother outside and/or inside a vertical common path. Several optionsalso exist as to the feeding of the helical entries of these dies, e.g.central feeding of all layers or side feeding of the layers withhorizontal split feed at different heights. Examples are to be found inU.S. Pat. No. 6,702,563

Modular plate mandrels are split two-part modules which, like someconical designs, stack one on another. There are basically two options;out-in versions in which the melt streams flow from outside to inside,and in-out versions in which the melt streams flow from inside tooutside of each module. A combination of the two types is also possible.The melt distribution is typically horizontally split but it can alsocombine some vertical paths to reduce diameter.

It is also possible to combine the types, for example the basic diebeing of concentric mandrel type with some layers being of modular platedesign, usually for the outer layers of the multi-layer blown film.

WO01/78966 discloses a co-extrusion die with one of the extrudedcomponents being fed through a single side inlet into a bifurcated feedchannel which supplies the die outlet and similar constructions areshown in WO90/11880 and JP2011005824.

US2004/022886 discloses a single layer extrusion die with side inletsfeeding multiple bifurcated feed channels.

FIG. 1 attached shows a five-layer concentric mandrel die 1 with centralfeeding in accordance with prior art. The die shown in FIG. 1 has an1800 mm diameter, i.e. for the annular path 110 to which all layers flowafter they merge. This diameter also corresponds to the diameter of themandrels of the middle layer of the die. Since the die is of centre-fedtype, all the extruder inlets 401-405 (only two of which—401 and 405 areshown for simplicity) through which the molten thermoplastic is fed tothe die are placed close to the bottom of the die and are spread aroundthe perimeter. As can be seen, such dies have complex internalconstructions, requiring the accurate registration of components in thedifferent layers.

To avoid complexity in the figures only the main elements of parts ofthe flow paths are shown, but in detail each flow path includes:

-   -   a horizontal inlet part 401-405, which extends to the centre of        the die for the outer layer and towards the centre of the main        die body 10, but to an off-center point for the remaining        layers.    -   a path 501-505 directed upwards and making any necessary bends        in order to avoid collision with other layers and reach the        centre (only path 501 is shown—for simplicity).    -   multiple inclined radial ports 601-605 (which may also be        horizontal and only port 601 is shown). Each of the inclined        ports also has an additional vertical path as soon as it arrives        at the layer mandrel (the one indicated in the drawing is small        but still exists).

The length of each of the flow paths for the layers 201-205 from the dieinlet to helical outlet channel start point is, for the layers in turnfrom inner layer to outer layer, 2772 mm, 2776 mm, 2803 mm, 2834 mm and2893 mm.

Blown film dies exist from sizes of 50 mm to 2500 mm diameter. Most ofthe dies that are used for packaging film applications are of a maximumdiameter of 900-1000 mm and up to eleven layers. These dies are ofeither concentric mandrel, modular or conical, or mixed. Conical stackedmandrel and modular plate designs can be implemented up to a diametersize of 900 mm, flared from 600-700 mm. There exist dies of modularplate design which are flared to 1300 mm from 600-700 mm and which havevery long flaring melt flow paths.

Larger dies (up to 2500 mm) are typically of three to five layers andare usually central or annular mandrel dies, typically used foragricultural applications (e.g. greenhouse films) where large filmdimensions are necessary (e.g. 8 to 22 m bubble circumference, 100-200μm thickness), or for geomembrane applications (6 to 8 m bubblecircumference, 500-2500 μm thickness)

In existing concentric dies of side feed design, the material follows abinary split distribution feed channel arrangement from a single sideinlet of the layer to the starting point of each extrusion helicaloutlet. As the die gets bigger in diameter, the length of this flowchannel gets longer and longer. As a consequence, higher melt pressuresare developed in use and the material residence time gets longer,resulting in increased melt temperatures and material degradation. As aresult side feed concentric mandrel dies have been limited to about 1200mm die diameter.

In a typical large blown film die having 3 to 5 layers and 1800 mm diediameter all layers are centrally fed. For the middle layer of the die,this results an overall length between the die inlet to the helicaloutlet of more than 2700 mm (see above reference to FIG. 1).

As a consequence the average residence time as well as the tail of theresidence time distribution become very long. In addition the size ofthe die does not allow reducing the thickness of a specific layer whilemaintaining good thickness uniformity of this layer due to the very longpaths that the melt has to flow though within the die and the requiredlow material quantity for such layer. Further, the back pressuredeveloped between the layer inlet and the start of the helical outletbecomes very high, reducing the remaining available pressure which canbe used for the helical outlet section of the layer to improve thicknessuniformity due to the fact that total available pressure is limited.

Residence time and melt distribution around the die circumference isalso a very critical issue for large dies, especially for sensitivematerials because of carbon build up, high purging time, waste,deposits, etc. (slow moving particles are prone to degradation and longpurging time).

An example is a 2 m diameter, five layer concentric mandrel die wherethe middle layer is designed to extrude an ethylene vinyl alcoholcopolymer (EVOH) film at a very low output and percentage (e.g. lessthan 4%) and at a very good thickness tolerance distribution around thedie circumference.

Such materials need to be processed with a very short residence time andalso need to be used in very low percentages due to their significantlyhigher cost in relation to standard materials. As an example, EVOH has acost which is in the range of 5-6 times higher compared to Polyolefins,therefore in case of a film combining both materials, EVOH has to beused in small percentages in order for the film to be of reasonable costwhile maintaining the advantage that using EVOH has as to the barrierproperties it provides.

Thus, it is often desirable to reduce residence time distribution, tominimize wetted surface area (the area where the polymer comes tocontact with the metal), to minimize the melt volume inside the die, tooptimise back pressure, to avoiding overheating the die, to enable rapidpurging for efficient product change-over and reduction of resin waste,to eliminate flow lines in the final products, to eliminate meltfracture, interfacial instability, gels, black spots, carbon built up,etc., to improve operational flexibility in resin selection andprocessing parameters, to increase output levels and/or efficiency, toimprove thickness tolerance of each layer and total film thickness, toimprove film optics, and to achieve thermal isolation between layersespecially the ones with significantly different processingtemperatures.

The present invention targets, in particular, large co-extrusion blownfilm dies (with mandrel diameter above 1200 mm) for producing filmbubbles of large circumference (8 to 22 m).

SUMMARY OF THE INVENTION

According to the present invention there is provided a concentricco-extrusion die having a plurality of annular or conical die mandrellayers, each layer comprising a pair of adjacent annular or conical diemandrels defining between them a flow path for molten thermoplasticsmaterial from an inlet to an annular extrusion outlet through which athermoplastics tubular extrusion is formed in use, extrusion through themultiple annular or conical layer outlets forming a multi-layeredproduct, characterised in that

-   -   at least one layer of the annular or conical die mandrels has a        plurality of molten material inlets arranged around the external        circumference of the co-extrusion die, each inlet being        connected to a feed channel which has plural bifurcations        providing 2^(n) subsidiary outlet feed channels where n is the        number of bifurcations, and each subsidiary outlet feed channel        being connected to a corresponding helical outlet channel.

Such a construction is especially suited to dies having an annular filmoutput of diameter above 1200 mm.

The molten material inlets arranged around the external circumference ofthe co-extrusion die may be connected to the corresponding feed channelsvia respective inlet paths passing through a main body of the dieseparate from the mandrels.

Alternatively, the molten material inlets arranged around the externalcircumference of the co-extrusion die may be connected to thecorresponding feed channels via respective inlet paths passing through acentral die block separate from both the mandrels and from a main bodyof the die.

The present invention can be implemented for a concentric mandrel die orconical central fed die or a combination of these.

It has been found that this significantly shortens the flow path lengthbetween the layer inlet and the start of the helical outlet channelcompared to the corresponding flow path length for the same layer of thesame size in a conventionally centrally fed concentric mandrel die aswell significantly shortens residence time in the die. In additionpressure drop is minimised between the layer inlet and the start of thehelical outlet channels compared to the pressure drop across thecorresponding section of the same layer of a same size centrally fedconcentric mandrel die. The reduction in pressure allows the gain inpressure to be used or partially used in the helical section of thelayer to improve thickness uniformity. This results in bettercontrolling of the uniformity of thickness of this layer by optimisingthe distribution of the melt flow around the circumference of the die.

In addition, although the presence of multiple side entry inlets maycomplicate the construction of the die as the side inlets have to passthrough multiple other layers of the die, avoiding interference withchannels in the other layers, and hence is counter-intuitive, due to theperipheral feed of the material and the shorter flow paths the melt isdistributed around the circumference in an optimal way resulting inuniform thickness distribution even at very low percentages and outputsof the respective layer(s).

These factors are particularly important when using certainthermoplastic materials such as EVOH, Polyamide, PVDC and fluoropolymersas explained above, and particularly in a composite, multi-layer filmwhere obtaining the right matching conditions between the films is alsoimportant.

In a die of the invention, the total number of layers of the die can befrom two to twenty one and the number of side feed layers from one toeleven correspondingly. The side fed layers can have a plurality ofmolten material inlets with bifurcated feed channels disposed over apart or all of their length in one or more of a vertical, horizontal orconical orientation according to the type of mandrel used. The remaininglayers can have central or annular feeding.

The number of inlets of the said layers may be between 2 to 16.

These types of dies can be installed in either a blow up configurationor in a blow down configuration.

It is also possible to have different materials for each of theextruders supplying the mandrel, resulting in different propertiesaround the circumference of the film and resulting bubble.

Another possibility is to improve the thickness tolerance by controllingseparately the throughput of each extruder supplying said mandrel, andin this way to correct thickness deviations around the bubblecircumference. This can be done with or without melt pumps and can beconnected with a thickness measurement unit for on-line thicknesscontrol.

The invention includes a method of extruding a multi-layer thermoplasticfilm using a co-extrusion concentric die as defined above. Material ofparticular interest which can be extruded through the at least one layeraccording to this method may be comprised of a polyamide or ethylenevinyl alcohol copolymer (EVOH) or PVDC or fluoropolymers. Additionalmaterials of particular interest for this method are ThermoplasticPolyurethanes (TPU) or Polybutene-1 (PB-1).

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of multi-layer concentric annular dies according to the priorart and the invention are shown in the accompanying drawings, in which:

FIG. 1 (Prior art) is a longitudinal section of a prior art design of afive-layer concentric annular extrusion die;

FIG. 2 (Prior Art) is a longitudinal section through a conventional sidefed co-extrusion die of relatively small dimensions;

FIG. 3 (Prior Art) is a longitudinal section of a conventional design ofa seven-layer concentric annular extrusion die;

FIG. 4 has a main view which is a longitudinal section of a firstexample of a five-layer concentric annular co-extrusion die with themelt flow path of the middle extrusion outlet layer (3^(rd) layer) , anda secondary view showing a developed partial annular section through themiddle layer, in accordance with the invention;

FIG. 5 shows, plan view, an arrangement of extruders around theco-extrusion die of FIG. 4;

FIG. 6 is similar to FIG. 4, but showing another co-extrusion die withthe melt flow path of the 5^(th), outer, layer and developed, inaccordance with the invention;

FIG. 7 is a plan view, similar to that of FIG. 5, but showing anarrangement of extruders around a seven-layer extrusion die;

FIG. 8 shows the corresponding longitudinal and developed sections forthe co-extrusion die of FIG. 7;

FIG. 9 shows another embodiment of the corresponding longitudinal anddeveloped sections for the co-extrusion die of FIG. 7;

FIG. 10 shows a top view of a modular plate section providing the outerlayer of a concentric die;

FIGS. 11A-11F are partial cross-sectional views of parts of respectiveconcentric dies to illustrate the form and position of the channelsrelative to die components; and

FIGS. 12 and 13 illustrate a multi-layer concentric die in longitudinalsection and plan view respectively, with two side-fed layers being fedfrom the same extruders.

DETAILED DESCRIPTION OF EXAMPLES IN ACCORDANCE WITH THE INVENTION

In the examples described below, the mandrel exit diameter is shown tobe 1800 mm. However co-extrusion dies according to the present inventioncan have any desired exit diameter, but are especially suitable fordiameters in the range of 1300 to 2500 mm. The detailed design of allparts depends on the final application as well as required residencetime, pressure and other rheological parameters.

In the deextruders, total number of layers, number of layers which areside fed with multiple inlets, number of inlets per layer, number ofbifurcations, number of spirals overlaps, number of spirals, number ofradial ports, inclination of ports, inclination of side feed inlets etc)are indicative and only for the purpose of the examples.

In FIG. 4, a longitudinal section of a first concentric co-extrusion dieis shown as well as a developed partial annular section of the middlelayer. The co-extrusion die 1 has six concentric mandrels 101-106 whichdefine between them five annular extrusion layers 201-205, the outlets301-305 of which feed an annular exit 110. The die mandrels 102,103 aresupported on a main die body 10, part of which forms the die mandrel101, whereas the die mandrels 104,105,106 are supported on a central dieblock 11 as described in more detail below.

The layers 201,202,204 & 205 have conventional feed channel arrangementsin which the melt of each layer follows a generally horizontal inletpath, 401,402,404,405 respectively through the main die body 10 (notethat because of the position of the section the inlet paths 402 & 404are not shown in FIG. 4) towards the central axis 20 of the die and eachof which paths is then directed upwardly (as shown) along feed channels501,502,504,505 (making any necessary bends in order to avoid collisionwith the melt stream flow paths of other layers). At the end of the feedchannels where the respective feed channel arrives at the centre of thedie, it splits into a number of radial channels, 601,602,604,605respectively, typically 16 for each layer, directed from the centre tothe periphery of the main die body 10. Only one of these radial channelsfor each layer is shown in FIG. 4.

The outlets of the radial channels are evenly distributed around themain die body leading into the respective outer circumference of therespective mandrel.

The radial channels 601 and 602 extend to the respective mandrel of thecorresponding layer, where they are further split into two (not shown),and each such split feeds a single helical outlet channel 701,702respectively, of the layer, so that each radial channel supplies twohelical outlets. The helical outlets feed respective annular channels801,802 which in turn feed inclined conical outlet channels 901,902which extend to the annular die exit 110 as shown, where, in operation,all the individual layers of the film are brought together in the finalannular extrusion. The channels 604,605 on the other hand take aslightly different form as they extend to a central die block 11, atwhich point they turn so as to lie generally parallel to the die axis 20before entering into the corresponding die layers 204, 205. In otherrespects their paths are similar to the radial channels 601,602. At therespective mandrel of the corresponding layer, they are further splitinto two (not shown), and each such split feeds a single helical outletchannel 704,705 respectively, of the layer, so that each radial channelsupplies two helical outlets. In an alternative form, the channels604,605 may be split within the central die block 11, with or withoutfurther splits occurring within the mandrels. The helical outlets feedrespective annular channels 804,805 which in turn feed inclined conicaloutlet channels 904,905 which extend to the annular die exit 110 asshown, where, in operation, all the individual layers of the film arebrought together in the final annular extrusion.

However, layer 203 is fed, in accordance with the present invention,from the side, as indicated in the FIG. 4, through four horizontal inletfeed channels 403 (only one of which is shown in the section of FIG. 4),extending directly from the external circumference 30 of the die througha central die block 11 until it reaches the outer annular surface ofmandrel 103 corresponding to the third or middle layer 203. Afterreaching the surface of mandrel 103, each feed channel 403 is bifurcatedthree times (as shown in the developed section of FIGS. 4 at 403.1,403.2, & 403.3) so that, ultimately, each feed channel 403 feeds eightsubsidiary feed channels 503, each of which in turn feeds a respectivehelical outlet 703 of the layer 203, there thus being 32 in total forthe layer 203. The central die block 11 which is formed as a singleannular component, interfaces with the lower annular surfaces of themandrels 104,105,106 to support them and also with the externalcircumferential surface of the mandrel 103 and provides a route for thefeed channels 403 which avoids the need for complex sealing and/orregistration between the mandrels 104,105,106 were the feed channels tohave to pass directly through each of those mandrel layers. The centraldie block 11 thus also effectively shortens the die mandrels 104-106,

The bifurcated distribution of a single one of the four feed channels403 is indicated in the developed partial annular section of FIG. 4. Intable 2 below, the length between the inlets of the middle layer to thedie and the entry 403.3 of the last bifurcation before the inlet of thecorresponding helical outlets has been calculated. For the middle orthird layer and it is 940 mm. This example can be compared to the priorart die of FIG. 1 where the length of the flow path of the same middlelayer of the same diameter die is calculated as 2803 mm. As one can see,with the length of feeding path of the central layer of the die fedaccording to prior art being 2803 mm as indicated in table 1, while thelength of the same path according to the present invention is 940 mm (asindicated in table 2 below), there is a 66.4% reduction in overalllength. Given that the path length has very important impact onresidence time and back pressure, such a reduction will also reduce theresidence time and back pressure, thus enabling the use of heatsensitive materials in dies (especially larger ones).

In addition, having less pressure drop in the distribution section ofthe flow path enables more pressure to be available to be consumed(totally or in part) in the helical outlet channels 701-705. Thisimproves thickness uniformity. If the pressure margin is only partiallyconsumed at the helical outlets or if it is not consumed at all, reducedbackpressure at the die inlet results, which is also an advantage, sincereduced pressure also results in a reduction of the melt temperature.

TABLE 1 Layer length calculation of a prior art 5 layer centrally fed coextrusion die Layer Diameter Total length A (inner) 1480 2772 B 16402776 C(middle) 1800 2803 D 1960 2834 E (outer) 2120 2893

FIG. 5 shows a top view of the concentric die extruder of FIG. 4 for afive layer blown film line. In this embodiment, the middle layer 203 ofthe five layer line extrusion die 1 has side feed from four smallextruders 2, while the remaining layers 201,202,204,205 are eachcentrally fed each by a single larger extruder, 3.

While four extruders, one corresponding to each inlet channel 403 areshown in FIGS. 4 and 5, other numbers of extruders could be used inorder to feed the central or third layer 203 of the extruder. Forexample, two or three extruders can be used with a corresponding numberof inlet feed channels 403.

In addition, any layer could be side-fed supplied with any desirednumber of extruders, while also more than one layer (two, three etc)could be side fed simultaneously in accordance with the presentinvention. Further, the extrusion die can have any number of layersequal to or more than two and any diameter, especially above 1300 mm.

In other embodiments one extruder can feed more than one said layer fromeach side of the die.

In a further embodiment according to the present invention shown in FIG.6, outer layer 205 is side fed through feed channel 405′ which passesthrough the mandrel 106. In this case, referring to table 2 below, theflow path length is 874 mm. The length of flow path for the same layerof a die according to the prior art is 2893 mm as shown in the abovetable 1, therefore providing a 69.7% reduction in length according tothis example of the invention.

TABLE 2 Layer length calculations for a 5 layer co-extrusion die of theexample Layer Diameter Total length C(middle) 1800 940 E (outer) 2120874

In FIG. 7 another embodiment in accordance with the present invention isshown. In this case, we have the top view of a seven layer die. In thisdie layers 201,202,203,205,206,207 are each centre fed by a respectiveextruder 3. Layer 204 is side fed by four smaller extruders 2.

FIG. 8 shows a first embodiment of the longitudinal and developedsections of the 1800 mm die of FIG. 7. Melt feed to the layer 204 isimplemented with four inlets 404 in accordance with the presentinvention, with the inlet channels 404, like the inlet channel 403 ofFIG. 4, passing through a central die block 11 directly to the layer204. The length calculated is 1020 mm as shown with reference to table 3below. Comparing this to the same layer of the corresponding prior artdie design, we can see that for a die design implemented according toprior art the corresponding length is 2968 mm as shown in table 5 below.Therefore, according to the present invention, we have a lengthreduction of 65.6%.

TABLE 3 Layer length calculations for a 7 layer co-extrusion die of theexamples Layer Diameter Total length D(middle) 1800 1020 G (outer) 2280922

FIG. 9 shows another embodiment of the longitudinal and developedsections of the 1800 mm die of FIG. 7. Melt feed to the layer 204 isimplemented with four inlets 404 in accordance with the presentinvention entering the die directly through the main body 10 of the dieand then turning to be parallel with the die axis 20 before entering themandrel. To allow machining of the parallel portion 404′ it is drilledfrom the lower surface 12 of the main die body 10 and the lower part404″ is then closed by a plug. The length calculated is 1320 mm as shownwith reference to table 4 below. Comparing this to the same layer of thecorresponding prior art die design, we can see that for a die designimplemented according to prior art the corresponding length is 2968 mmas shown in table 5 below. Therefore, according to the presentinvention, we have a length reduction of 55.5%. In a further embodiment(not shown) the radial part of the channel inlet 404 may extend though apipe to the bottom of the channel portion 404″ rather than through themain die body 10.

TABLE 4 Layer length calculations for a 7 layer co-extrusion die of theexamples Layer Diameter Total length D(middle) 1800 1320 G (outer) 2280922

For a corresponding seven layer central fed concentric die of prior artdesign shown in FIG. 3 (of 1800 mm die diameter) the lengths of each ofthe melt flow paths is indicated in table 5 below.

TABLE 5 Layer length calculation of a prior art 7 layer centrally fed coextrusion die design Layer Diameter Total length A (inner) 1320 2999 B1480 2902 C 1640 2936 D(middle) 1800 2968 E 1960 3023 F 2120 3119 G(outer) 2280 3087

Comparing tables 3 and 4 with table 5, we can conclude that for theouter layer G we also have a significant reduction in length by 2165 mmor 70.1%

The feed to other layers of the die shown in FIGS. 7, 8 and 9 can alsobe implemented in a similar way if desired.

Calculations in respect of a known 450 mm diameter five layer concentricmandrel die (shown in FIG. 2) with side feeding from a single inlet andbifurcated feed channels have also shown the flow path length for thecentral layer to be 1141 mm. This is compared with the 1800 mm die ofFIG. 4 which shows a flow path length of 940 mm, Thus, even a prior artconcentric die of much smaller diameter has longer flow paths than aco-extruder die of the invention.

In all embodiments the bifurcated channel can be disposed, over a partor all of its length, in one or more of a vertical, horizontal orconical orientation, depending on die construction and position asillustrated, by way of example, in FIGS. 11A to 11F. Furthermore, agiven bifurcated channel BC can be positioned either wholly within thebody of one of the die mandrels M, at the surface of a particularmandrel M (see for example FIGS. 11B, 11C and 11F) or partly within eachof two adjacent mandrels M of the die which can be oriented vertically,horizontally or conically (see for example FIGS. 11A, 11D and 11E). Inaddition, combinations of the above positioning of the channel can beimplemented for example the bifurcated channel could be positionedpartly within the body of one of the die mandrels over a part of itslength and partly within each of two adjacent mandrels over a differentpart of its length. Any such combination is possible depending onrequirements. Also, the bifurcated channel may be of differentcross-sectional shapes, as shown in FIGS. 11A to 11F. Thecross-sectional shape of the feed channels can be circular, oval or anyother shape which can be machined and may vary over its length.

It is possible to have one or more modular plate die sections in theouter layer(s) of a concentric die in accordance with the invention. InFIG. 10 such a design is shown, with a modular plate 6 of 1800 mm. Inthis embodiment, four inlets 403 are provided which are bifurcated in asimilar manner to the examples above and in this case the flow pathlength to the end of the 32 helical outlet channels 703 is 1056 mm. Soagain, a reduction in flow path length is possible and similar to thatachieved in the previous examples.

It is to be noted that all lengths mentioned in the above examples andtables are indicative and they can vary according to the detail design.However in all cases the lengths of a die implemented according to thepresent invention are much shorter compared to a prior art die ofcomparable size and number of layers.

In a further embodiment shown in FIG. 12, one or more of the extruders 2can be arranged to supply melt to more than one layer. For example thiscan be achieved by splitting the output channel 21 of an extruder 2 intotwo output channels 213,215, each of which connects to and supplies adifferent die layer 207,204. In this case it is possible (but notnecessary) to use melt pumps 22 for exact control of the flow to eachinlet. FIG. 13 illustrates the arrangement as a top plan view forfurther understanding.

Further it is also possible that one single inlet, e.g. 405′ can bearranged to supply more than one layers of the die, e.g. 204 and 205.

Further, it is also possible that the exit of a bifurcated feed channel,e.g. 403.1, 403.2, or 403.3, is arranged to supply the helical channelsof more than one layer.

Although the figures of the various examples of the invention show thatthe bifurcations of the inlet channels extend mainly in planes normal tothe axis 20 of the dies, it is possible for the bifurcation branches toextend at least partially in the axial direction of the die, i.e. sothat they are either substantially parallel with the axis 20 or elseangled to it.

Table 6 below illustrates a comparison of pressures and residence timesbetween a prior art die and one of similar size but in which the middlelayer is implemented according to the present invention. Specifically,the pressures and residence times have been calculated for the middlelayer of a prior art 7 layer concentric centrally fed co-extrusion diedesign of 1800 mm diameter. These pressures and residence times havebeen calculated for three different materials Linear Low densitypolyethylene (LLDPE), Polyamide (PA) and Ethylene vinyl Alcohol (EVOH).In addition, two levels of shear rate have been considered, 10-13s⁻¹ and15 s⁻¹. The same calculation has been repeated for a 7 layer die of 1800mm diameter in which the middle layer has been implemented to be sidefed from four inlets according to the embodiments presented in the FIGS.8 and 9 . As the table indicates, an improvement from 50 to 60% in theside fed layer can be achieved for both pressure and residence time incomparison to a prior art centrally fed die.

TABLE 6 COMPARISON EXAMPLE INDICATING THE PRESSURE DROP AND RESIDENCETIME IMPROVEMENT DIE DIMENSIONS AND LAYER PERCENTAGES USED IN THEEXAMPLE Die gap diameter (mm): 1800 Number of layers 7 Output (Kg/h):1800 Film total thickness (μm): 180 Die layers location inner middleouter Die layers number 1 2 3 4 5 6 7 Die layers code A B C D E F GLayer percentage - example 18.33% 15.00% 15.00% 3.33% 15.00% 15.00%18.33% Middle layer output (Kg/h): 60 Middle layer thickness (μm): 6Comparison res

Data for Middle layer centrally fed (prior art) Middle layer side fedwith 4 inlets (% in respect t

middle layer Total Total prior art) Melt Total pressure Residence ShearTotal pressure Residence Shear Pressure Resid

temp length drop time Rate length drop time Rate drop tim

Material (° C.) (mm) (bar) (sec) (1/s) (mm) (bar) (sec) (1/s) (%) (%

Below is a simulation (based on the Carreau model) of the embodimentshown in FIG. 8 LLDPE 220 2968 283 124 13 1020 112 49 13 60.4 60.

2968 327 113 15 1020 130 45 15 60.2 60.

PA 230 2968 138 150 10 1020 55 60 10 60.1 60.

2968 227 115 15 1020 90 46 15 60.4 60.

EVOH 210 2968 290 156 10 1020 115 62 10 60.3 60.

2968 447 120 15 1020 178 48 15 60.2 60.

Below is a simulation (based on the Carreau model) of the embodimentshown in FIG. 9 LLDPE 220 2968 283 124 13 1320 140 62 13 50.5 50.

2968 327 113 15 1320 162 56 15 50.5 50.

PA 230 2968 138 150 10 1320 68 74 10 50.7 50.

2968 227 115 15 1320 112 57 15 50.7 50.

EVOH 210 2968 290 156 10 1320 143 77 10 50.7 50.

2968 447 120 15 1320 221 59 15 50.6 50.

indicates data missing or illegible when filed

Carreau is a well known rheological model which is used for simulatingthe rheological behavior of melt plastics. The ‘layer percentage’ shownin the first part of the table for each layer is the percentage ofmaterial by (presumably) weight.

Further simulations have shown that it is possible to run EVOH in themiddle layer of a 1800 mm die configured with four (4) extruders feedingthe die from the side as proposed by the present invention, andachieving output down to 33 kg/h with process conditions (shearstresses, shear rate, residence time, etc) according to raw materialsuppliers recommendations.

1. A concentric co-extrusion die comprising: an annular extrusion outlethaving a diameter equal to or greater than 1300 mm and having aplurality of die mandrel layers having a pair of adjacent annular orconical die mandrels defining between them a flow path for moltenthermoplastics material; and an inlet to the annular extrusion outletthrough which a thermoplastics tubular extrusion is formed in use,wherein at least one of said layers has a plurality of molten materialinlets arranged spaced apart around an external circumference of aco-extrusion die, each molten material inlet being connected to a feedchannel which has a plurality of bifurcations providing 2^(n) subsidiaryoutlet feed channels with n being the number of bifurcations, eachsubsidiary outlet feed channel being connected to a correspondinghelical outlet channel, and wherein extruding the molten materialthrough the at least one of said layers forms a multi-layered product.2. A concentric co-extrusion die according to claim 1, wherein themolten material inlets arranged around the external circumference of theco-extrusion die are connected to the corresponding feed channels viarespective inlet paths passing through a main body of the die separatefrom at least some of the mandrels.
 3. A concentric co-extrusion dieaccording to claim 1, wherein the molten material inlets arranged aroundthe external circumference of the co-extrusion die are connected to thecorresponding feed channels via respective inlet paths passing through acentral die block separate from the mandrels and from a main body of thedie.
 4. A concentric co-extrusion die according to claim 1, wherein eachof said layers has a plurality of molten material inlets arranged aroundthe external circumference of the co-extrusion die.
 5. A concentricco-extrusion die according to claim 4, wherein each of said layershaving a plurality of molten material inlets has the same number ofinlets.
 6. A concentric co-extrusion die according to claim 4, whereinone or more of said layers has a different number of inlets from one ormore of the others of said layers.
 7. A concentric co-extrusion dieaccording to claim 1, wherein the annular die mandrel layers arecylindrical or conical.
 8. A concentric co-extrusion die according toclaim 4, wherein the number of bifurcations in each of said layershaving a plurality of molten material inlets is the same in each saidlayer.
 9. A concentric co-extrusion die according to claim 4, whereinthe number of bifurcations in one or more of said layers having aplurality of molten material inlets is different from the number ofbifurcations in one or more of the others of said layers.
 10. Aconcentric co-extrusion die according to claim 1, wherein each of saidlayers having a plurality of molten material inlets has bifurcated feedchannels disposed over a part or all of their length in one or more of avertical, horizontal or conical orientation.
 11. A concentricco-extrusion die according to claim 10, wherein the orientation of thebifurcated feed channels in all said layers is the same.
 12. Aconcentric co-extrusion die according to claim 10, wherein one or moreof said layers has a different orientation of the bifurcated feedchannels from one or more of the others of said layers.
 13. A concentricco-extrusion die according to claim 1, further including an additionallayer comprising a modular plate having a plurality of inlets for thesupply of molten polymer, the modular plate being arranged to providethe an outer layer of extruded film.
 14. A concentric co-extrusion dieaccording to claim 13, further including a plurality of modular platesarranged to provide a plurality of outer layers.
 15. A concentricco-extrusion die system comprising of: co-extrusion die, the die havingan annular extrusion outlet having a diameter equal to or greater than1300 mm and having a plurality of die mandrel layers having a pair ofadjacent annular or conical die mandrels defining between them a flowpath for molten thermoplastics material; and an inlet to the annularextrusion outlet through which a thermoplastics tubular extrusion isformed in use, wherein at least one of said layers has a plurality ofmolten material inlets arranged spaced apart around an externalcircumference of a co-extrusion die, each inlet being connected to afeed channel which has a plurality of bifurcations providing 2^(n)subsidiary outlet feed channels, with n being the number ofbifurcations, wherein each subsidiary outlet feed channel is connectedto a helical outlet channel; and a plurality of thermoplastic materialextruders for supplying the at least one of said layers with moltenmaterial.
 16. A concentric co-extrusion die system according to claim15, in which each extruder supplies one or more inlets of one or morelayers of the die.
 17. A concentric co-extrusion die system according toclaim 15, wherein all or part of said extruders are connected to theinlets of the die via melt pumps.
 18. A method of extruding amulti-layer thermoplastic film using a co-extrusion concentric diecomprising the steps of: providing a co-extrusion die, the die having anannular extrusion outlet having a diameter equal to or greater than 1300mm and having a plurality of die mandrel layers having a pair ofadjacent annular or conical die mandrels defining between them a flowpath for molten thermoplastics material; and an inlet to the annularextrusion outlet through which a thermoplastics tubular extrusion isformed in use, wherein at least one of said layers has a plurality ofmolten material inlets arranged spaced apart around an externalcircumference of a co-extrusion die, each inlet being connected to afeed channel which has a plurality of bifurcations providing 2^(n)subsidiary outlet feed channels with n being the number of bifurcations,each subsidiary outlet feed channel being connected to a helical outletchannel; supplying the at least one of said layers with molten material;and extruding the molten material through the at least one of saidlayers to form a multi-layered product.
 19. A method according to claim18, wherein the material extruded through the at least one layer isselected from the group consisting of a polyamide, ethylene vinylalcohol copolymer (EVOH), polyvinylidene chloride (PVDC), and afluoropolymer.
 20. A method according to claim 18, wherein the materialextruded through the at least one layer is comprised of a ThermoplasticPolyurethane (TPU) or Polybutene-1 (PB-1).